The present invention concerns photographic copying or printing machines of the type provided with a carrier or support for originals to be copied (e.g., negatives to be printed), the carrier being provided with an exchangeable format mask for different formats of originals to be copied, a variable objective being provided to image the original onto the copy-medium plane, and the copying machine being provided with an automatic exposure control system which automatically sets the exposure time to be implemented for different constituent colors in dependence upon the at least one photodetector receiving light transmitted through the oringinal.
Photographic copying machines are often provided with exchangeable format masks to accommodate negatives of differing dimensions and/or to frame sections of negatives to be copied for differing enlargement of such sections. It is of course also known to provide such copying machines with automatic exposure control systems comprising photodetectors which generate signals dependent upon the density and/or color balance of the originals and means automatically setting the intervals of exposure time which are to be allotted for the component colors involved.
For machines provided with automatic exposure control and exchangeable format masks, the size of the negative or format-mask aperture involved is an important parameter for the automatic exposure control system of the machine.
For example, in Federal Republic of Germany Pat. No. 1,209,871, each format mask is provided with markings identifying the size of the mask aperture, these markings being provided in the form of depressions or apertures sensed by the pins of spring-contact switches, to inform the exposure control system of the machine what negative size will be involved. Federal Republic of Germany published patent application (Offenlegungsschrift) 26 59 449 discloses providing each format mask with magnetic elements which is coded form indicate the respective format size. These magnetic elements control magnetically responsive switching elements such as reed contacts, Hall elements, or the like.
The disadvantage of such prior-art approaches is that when a large number of different format masks are involved it is necessary that a sizable encoding capacity be provided, which in turn requires a correspondingly large number of code-reading and code-representing elements, such as the aforementioned Hall elements, spring-contact switches, or whatever. In addition, appropriate decoding circuitry must be provided.